Si-hong Liu,Erich Buer

aCollege of Water Conservancy and Hydropower Engineering,Hohai University,Nanjing 210098,China

bInstitute of Applied Mechanics,Graz University of Technology,Graz 8010,Austria

Preface for special section on long-term behavior of dams

Si-hong Liua,Erich Bauerb

aCollege of Water Conservancy and Hydropower Engineering,Hohai University,Nanjing 210098,China

bInstitute of Applied Mechanics,Graz University of Technology,Graz 8010,Austria

When it comes to the long-term operation of hydropower plants,the service life of mechanical and electrical equipment and the reliability of hydraulic engineering structures are both crucial.The loading history and interaction of the construction with ground settlements,earthquake activities and flooding,chemical reactions with water,overtopping,and the state of weathering and aging of the construction material may play an important role in the safe operation of hydraulic structures such as earth,rock fill,and concrete dams,as well as novel reinforced soil structures.A critical reduction of the safety of dams can be caused by the evolution of internal erosion,cracksin the sealing,earthquake-induced deformations,reduction of the slope stability of earth and rock fill dams triggered by heavy rainwater inf i ltration,or rapid reduction of the water level in the reservoir.In order to take into account the complex interaction of different factors,the relevant aspects must be treated in an appropriate way by the correspondingdisciplines.In other words,multilateral cooperation is required between the disciplines of material technology, mechanics of materials,structural analysis,geotechnics and hydraulic structures,and environmental engineering.To this end a series of international conferences and workshops, including the First International Conference on Long Time Effects and Seepage Behavior of Dams(LTESBD08,Zhu et al., 2008),the Second International Conference on Long Term BehaviourofDams(LTBD09,Baueretal.,2009),andtheThird International Workshop on Long-Term Behaviour and Environmentally Friendly Rehabilitation Technologies of Dams (LTBD2015,LiuandBauer,2015),wereinitiatedtopresentand exchange experience and the latest developments related to the design,performance,rehabilitation,and environmental aspects of earth,rockfill,and concrete dams.LTBD2015 was held at Hohai University in Nanjing,China,in 2015.The organizers of this workshop invited authors of selected contributions to submit the extended versions of the papers that were presented. As a result,eight articles have been accepted for the present special issue.The main content of the papers is summarized below.

In the study by Alonso and Pinyol(2016),numerical simulations were carried out to investigate the inf l uence of a rapid reduction of the water level in the reservoir on the stability of an initially fully saturated slope of an earth dam.For the analysis of the safety of the slope against failure,different approaches for the prediction of the water pressure distribution during and after drawdown were evaluated.In this context,the correct description of the permeability plays an important role. It is stated in the paper that models based on simplified assumptions such as pervious and rigid soil or,alternatively, fully undrained conditions,are not recommended because they may underestimate or overestimate the distribution of the pore water pressure.For more accurate and ref i ned modeling,a coupled hydro-mechanical description of the problem is suggested and validated for two case studies.

Sun et al.(2016)discuss the decrease of the safety factor of slope stability during a rainfall inf i ltration event.The mechanical behavior of the unsaturated soil slope is described with a liquid-gas-solid three-phase model.For the numerical procedure,a methodology that loosely couples the liquid-gas two-phase flow and the deformation of the solid phase is used. The numerical results show that the decrease of the safety factor does not necessarily stop immediately when the rainfall stops.

The modeling of the seismic response of rockfill dams is still a challenging task,as discussed in the paper by Chen et al.(2016).To model the mechanical behavior of the rockfill material under cyclic loading,the assumption is made that the strain history can be decomposed into a cyclic part described by visco-elastic behavior and a permanent plastic shear strain described by an empirical formula depending on the shear strain amplitude,the mean stress,and the number of cycles.Numerical simulations were performed to analyze the seismic response of the Zipingpu concrete face rockfill dam in China.

Cen et al.(2016)numerically investigated the damage of concrete slab elements of a 100 m-high concrete face rockfill dam during earthquakes with different seismic intensities.Tostudy the process of cracks growing,a concrete random mesoscopic damage model was used in the three-dimensional (3D) fi nite element simulation.

Tschernutter and Kainrath(2016)discuss basic design criteria for small dams with a central reinforced concrete diaphragm as a sealing blanket.From an economic and technical point of view,such small dams,with a maximum height of about 40 m,can be an alternative to conventional designs for permanent impounded reservoirs or retaining dams for flood protection.

Liu et al.(2016)present the results of a numerical stressdeformation analysis of a rockfill dam with a clay-core wall built on a thick layer of overburden sediments and a concrete cut-off wall in the dam foundation.It is concluded that an appropriate connection between the clay-core wall and the concrete cut-off wall is needed and concrete with a low stiffness and high strength should be used to reduce the compression stress in the cut-off wall.

Li et al.(2016)present two different constitutive models to describe the behavior of weathered and creep-sensitive rockfill materials.Time-dependent material properties like creep are taken into account in a model based on the concept of generalized plasticity as well as a model based on the concept of hypoplasticity.In both models,the solid hardness is a key parameter for ref l ecting the state of weathering.The timedependent process of degradation of the solid hardness is described by an evolution equation.The performance of the models is demonstrated by comparing numerical simulations with experiments.

In the paper by Wu et al.(2016),the behavior of the Jinping-Iconcrete arch dam in China during initial impounding is analyzed.With a maximum height of 305 m,it is currently the highest completed dam in the world.The safety of the dam during the four impounding phases is analyzed twice based on the data from monitoring and numerical computation.

This special issue was initiated by the guest editors based on scientif i c and technical cooperation between Hohai University in Nanjing,China,and Graz University of Technology,in Graz, Austria.We extend sincere thanks to all who contributed to this issue,for which the manuscripts were reviewed according to the rules of Water Science and Engineering.

Alonso,E.E.,Pinyol,N.M.,2016.Numerical analysis of rapid drawdown: Applications in real cases.Water Sci.Eng.9(3),175—182.http:// dx.doi.org/10.1016/j.wse.2016.11.003.

Bauer,E.,Semprich,S.,Zenz,G.,eds.,2009.Proceedings of the 2nd International Conference on Long Term Behaviour of Dams(LTBD09).Verlag der Technischen Universit¨at Graz,Graz.

Cen,W.J.,Wen,L.S.,Zhang,Z.Q.,Xiong,K.,2016.Numerical simulation of seismic damage and cracking of concrete slabs of high concrete face rockfill dams.Water Sci.Eng.9(3),205—211.http://dx.doi.org/10.1016/ j.wse.2016.09.001.

Chen,S.S.,Fu,Z.Z.,Wei,K.M.,Han,H.Q.,2016.Seismic responses of high concrete face rockfill dams:A case study.Water Sci.Eng.9(3),195—204. http://dx.doi.org/10.1016/j.wse.2016.09.002.

Li,L.K.,Wang,Z.J.,Liu,S.H.,Bauer,E.,2016.Calibration and performance of two different constitutive models for rockfill materials.Water Sci.Eng. 9(3),227—239.http://dx.doi.org/10.1016/j.wse.2016.11.005.

Liu,S.,Bauer,E.,eds.,2015.Book of Abstracts for the 3rd International Conference on Long-Term Behaviour and Environmentally Friendly Rehabilitation Technologies of Dams(LTBD2015).Verlag der Technischen Universit¨at Graz,Graz.

Liu,S.H.,Wang,L.J.,Wang,Z.J.,Bauer,E.,2016.Numerical stressdeformation analysis of cut-off wall in clay-core rockfill dam on thick overburden.Water Sci.Eng.9(3),219—226.http://dx.doi.org/10.1016/ j.wse.2016.11.002.

Sun,D.M.,Li,X.M.,Feng,P.,Zang,Y.G.,2016.Stability analysis of unsaturated soil slope during rainfall inf i ltration using coupled liquid-gas-solid three-phase model.Water Sci.Eng.9(3),183—194.http://dx.doi.org/ 10.1016/j.wse.2016.06.008.

Tschernutter,P.,Kainrath,A.,2016.Design considerations and behavior of reinforced concrete core dams during construction and impounding.Water Sci.Eng.9(3),212—218.http://dx.doi.org/10.1016/j.wse.2016.11.006.

Wu,S.Y.,Cao,W.,Zheng,J.,2016.Analysis of working behavior of Jinping-I Arch Dam during initial impoundment.Water Sci.Eng.9(3),240—248. http://dx.doi.org/10.1016/j.wse.2016.11.001.

Zhu,Y.,Liu,S.,Qiang,S.,Chiu,A.,eds.,2008.Proceedings of the 1st International Conference on Long Time Effects and Seepage Behavior of Dams(LTESBD08).Hohai University Press,Nanjing.

E-mail addresses:sihongliu@hhu.edu.cn(Si-hong Liu),erich.bauer@ tugraz.at(Erich Bauer).

Peer review under responsibility of Hohai University.

http://dx.doi.org/10.1016/j.wse.2016.11.004

1674-2370/?2016 Hohai University.Production and hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license(http:// creativecommons.org/licenses/by-nc-nd/4.0/).